Observation of aJPC=1−+Exotic Resonance in Diffractive Dissociation of190   GeV/c

Abstract

The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the π- π- π+ final state using a 190 GeV/c pion beam hitting a lead target. A partial wave analysis has been performed on a sample of 420,000 events taken at values of the squared 4-momentum transfer t' between 0.1 and 1 GeV2/c2. The well-known resonances a1(1260), a2(1320), and π2(1670) are clearly observed. In addition, the data show a significant natural-parity exchange production of a resonance with spin-exotic quantum numbers J(PC)=1-+ at 1.66 GeV/c2 decaying to ρπ. The resonant nature of this wave is evident from the mass-dependent phase differences to the J(PC)=2-+ and 1++ waves. From a mass-dependent fit a resonance mass of (1660±10(-64)(+0)) MeV/c2 and a width of (269±21(-64)(+42)) MeV/c2 are deduced, with an intensity of (1.7±0.2)% of the total intensity.

Odd and even partial waves ofηπ−andη′π−

“…Fitted P -wave resonance masses in both channels are found to be shifted upwards by typically 200 MeV/c 2 when introducing constant-phase model backgrounds as in Ref. [23]. In the present Letter, we refrain from proposing resonance parameters for the exotic P -wave or even the exotic F and H -waves observed here.…”

“…For η π − , ambiguities occur when the PWA is extended beyond the dominant L = 1, 2 and 4 waves. We resolve this by requiring continuous behaviour of the dominant partial waves and of the Barrelet zeros [24]. The acceptable solutions agree within the statistical uncertainties with the solution selected here, which is the one with the smallest L = 3 contribution.…”

“…The data were subjected to a partial-wave analysis (PWA) using a program developed at Illinois and VES [21][22][23]. Independent fits were carried out in 40 MeV/c 2 wide bins of the four-body mass from threshold up to 3 GeV/c 2 (so-called mass-independent PWA).…”

“…In general, a two-pseudoscalar PWA suffers from discrete ambiguities [27,28,24]. The observed insignificance of unnatural-parity transfer crucially reduces the ambiguities.…”

Decays of tetraquark resonances in a two-variable approximation to the triple flip-flop potential

“…Furthermore they concluded that this particle could be the first radial excitation of the l ¼ 1 state of a cq " c " q state while they identified another particle, the Yð4350Þ as the ground state. Recently the COMPASS collaboration [12] reported the observation of 1 ð1600Þ, a resonance with spin-exotic quantum numbers J PC ¼ 1 Àþ that can only be explained by assuming a hybrid or tetraquark constitution.…”